In a wavelength division multiplexing optical transmission system that performs long-distance transmission of an optical signal having been subjected to wavelength division multiplexing (WDM), an amplification device that amplifies the optical signal through Raman amplification is introduced. The amplification device amplifies signal light by outputting Raman excitation light with a specific wavelength to a transmission line in order to compensate for a level reduction associated with transmission, and thereby extend relay distance. Note that the level reduction as mentioned herein refers to a reduction in the intensity of signal light according to the transmission distance. In addition, the transmission line is implemented by an optical fiber for transmitting signal light, and includes connecting points such as optical connectors. Excitation light for Raman amplification to be output to the transmission line typically has a level of several hundred mW. That is a level where damage to a transmission line is concerned when there is abnormality, such as contamination or flaws, in the transmission line. Specifically, when an impurity is attached to an end surface of an optical connector, the impurity becomes an absorber of light passing through the optical connector. Due to the absorption of the light, the temperature of the optical connector increases and accordingly, for example, the transmission line may be broken. When the damage occurs in the transmission line, replacement or fusion work of the transmission line is required, which involves much effort. In addition, in general, administrators of the transmission line and the amplification device are not always identical. Therefore, there is a problem that, when an optical connector in the transmission line side is damaged, it is not easy to replace the optical connector.
In order to solve such problems mentioned above, a technique using a measuring device, so-called an OTDR (Optical Time-Domain Reflectometer) device, that monitors a reflection state in a transmission line is recommended. In this technique, by using the OTDR device, the fact that abnormal reflection has not occurred in an optical connector is measured before a device is connected, by which it is verified that contamination is not attached.
However, the OTDR device may be expensive. Thus, it is realistically impossible to install the OTDR device for each of all excitation light source devices in terms of a total cost in a system. A method for sequentially measuring for each amplifier is rather realistic. Hence, instead of a connection mode of connecting the OTDR device with the excitation light source device at all times, another connection mode is employed at a connecting point of an optical connector of the excitation light source device to change over connections of a Raman amplifier and the OTDR device. Since such the mode is employed, there is a possibility that, when the OTDR device is connected to a Raman amplifier after the OTDR device measures whether there is abnormal reflection, a foreign matter may be attached to an optical connector.
A technique for addressing these problems is presented in a Patent Literature 1. The Patent Literature 1 teaches an optical amplifier employing a method for detecting the above-described transmission line trouble. This optical amplifier allows measurement light in addition to Raman excitation light to enter a transmission line to measure a connection loss based on the reflected light and backscattered light of the measurement light that occur in the transmission line, and controls the output level of the Raman excitation light according to the connection loss.